Sold as a science fiction film because it’s set a few years in the future, The Purge is in reality a grim, rather mean-minded home invasion thriller in the style of Straw Dogs or The Strangers and it’s never quite as compelling and gripping as it thinks it is because of the sheer lunatic implausibility of the sky-high central conceit which underpins its dark, claustrophobic narrative.

In 2022, we’re told, America has been reborn courtesy of its new ‘founding fathers’ who have established ‘the Purge’, a twelve-hour period across one night every year in which the rule of law and the emergency services are suspended and those with a criminal inclination can commit whatever crime they fancy – including mass murder – with absolutely no legal repercussions the next day or indeed ever. It’s a cathartic thing and as a consequence unemployment has tumbled and the country’s crime rate has dropped to 1%. Those who don’t feel the need to “release the beast” can barricade themselves in their fortified homes and wait until morning when normal service will be resumed. James Sandin (Hawke) has made a fortune selling home defence systems and he and his wife Mary (Headey) and their two teenage kids prepare to hunker down in their gated fortress and wait for the chaos of the Purge to pass. But inevitably a desperate refugee (Wakefield) gets into their house which is rapidly besieged by murderous outsiders and… yep, you guessed it, James and Mary face the prospect of becoming the very same amoral monsters they’re determined to keep outside as they try to keep their family together.

Unfortunately the idea of the Purge is so extreme and unlikely – it’s impossible to imagine any society being insane enough to enact such an implausible piece of legislation – it’s hard to really engage with the drama as it unfolds because your mind will be screaming “but this just couldn’t happen” even as guns are exploding, knives are buried in bellies and masked intruders prowl around the Sandin’s darkened home happy to murder anyone in their path just because they can get away with it. The Purge is essentially brutal and nasty, taking a pretty grim view of human nature and no one comes out of it especially well. The Sandins are a bit smug and mechanical (although Hawke and especially Headey put in strong, gutsy performances) and their attackers, many of them neighbours and acquaintances, aren’t convincing as murderers because there’s really no reason why they’d behave like this just for the hell of it. The Purge is a slick and oddly soulless affair, clearly out to make a point about the underlying violent nature of American society but you can’t help wondering if the point couldn’t have been made in a subtler and less extraordinarily unlikely story.